Heating, ventilating, and air conditioning (HVAC) systems are commonly used to ventilate enclosed spaces in structures, for example rooms in a building, and in particular those rooms occupied by people. Typically HVAC systems use ducting and at least one blower to deliver outside air to the interior spaces. Typical HVAC systems include blowers that operate at a constant RPM or output, and use dampers to control the airflow, frequently expressed as a volumetric airflow rate measured in cubic feet per minute, into a building. Although the blower may be located in various locations, commercial applications frequently use rooftop units with the blower located on the building's roof.
Many HVAC systems use one or more additional components in conjunction with the blower and ducting. For example, powered exhausts are used with HVAC systems to forcibly draw return air out of the building. Filters are also used frequently to remove certain components, such as dust, from the air. Dampers may also be used at various locations throughout the HVAC system to control the amount of air circulating in the system as a whole or in a portion of the system, such as in a particular room.
Economizers can be used to mix return air with incoming outside air to conserve energy and decrease operating costs. By mixing the return air with the incoming outside air, the incoming outside air is either pre-heated or pre-cooled, as appropriate, which decreases the energy required to heat or cool the air to the desired interior temperature. For example, when the outside air temperature is below the desired interior temperature, the HVAC system heats the incoming outside air and exhausts the warm return air outside the building. By using an economizer, the warm return air is mixed with and pre-heats the incoming outside air, thereby reducing the energy required to heat the mixed outside and return air to the desired temperature. However, the amount of return air that is mixed with the incoming outside air is frequently limited to limit the levels of harmful compounds in the building, such as CO2 or various organic compounds, thereby limiting the energy savings that may be realized with an economizer.
Energy recovery ventilators (ERVs) are also used with HVAC systems. The ERV is a heat exchanger that, similar to the economizer, either pre-heats or pre-cools the incoming outside air, as appropriate. However, in contrast with the economizer, an ERV mixes relatively little return air with the incoming outside air. One type of ERV uses a rotating disk arranged perpendicular to and bisecting two ducts, one duct for incoming outside air and the other duct for return air. The material in the rotating disk collects thermal energy from the duct with the warmer air and releases at least a portion of the collected thermal energy to the duct with the cooler air while minimizing mixing of the two airflows. Depending on the material used in the disk, some ERVs are also capable of transferring moisture (humidity) between the two ducts. HVAC systems must comply with various government and regulatory standards. One set of widely used standards are promulgated by the American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. (ASHRAE). These standards require a particular volumetric airflow rate of outside air entering a building for each building occupant, frequently measured in cubic feet per minute (c.f.m.). Current standards require a minimum of fifteen (15) cubic feet per minute of airflow entering a building for each occupant of the building.
After typical HVAC systems are installed in a building, trained technicians are required to “balance” the HVAC system. To balance a newly installed HVAC system, the technician determines the airflow required through a building's duct work by traversing or similar testing procedure and adjusts the airflow based on existing conditions. However, miscalculations, such as miscalculations of static pressure in the return air systems, can lead to improper airflow determinations. Additionally, the testing procedures typically fail to take into account overall system effect, such as the impact one airflow apparatus has on another airflow apparatus. Frequently, the overall system effect cannot be properly calculated when testing and balancing the entire system. For example, HVAC system components or subcomponents that operate intermittently during normal operation may be either continually on or continually off during the testing and balancing procedure. Furthermore, clogged filters, dirty ducts, and blower inefficiency due to long-term wear can degrade airflow through the duct work and result in the HVAC system delivering non-optimal airflow as the system ages.
Still other problems with prior HVAC systems include their inability to dynamically control the actual airflow entering a building. Since typical HVAC systems can not adjust airflow in response to, for example, actual building occupancy or CO2 levels, the actual “as installed” output of the HVAC system must be measured and set by the trained technician for an assumed building occupancy, which introduces additional errors and inefficiencies. For example, an installed HVAC system blower is typically set to deliver a high airflow rate sufficient to condition the building's interior when at or near a maximum expected occupancy during working hours, and reset to deliver a much lower airflow rate during non-working hours. Although this type of system maintains adequate airflow under normal conditions, under conditions where the actual occupancy differs from the estimated occupancy the actual airflow is either too high or too low. When the airflow is lower than required for existing conditions, the concentration of CO2 or other compounds may increase to unacceptable or dangerous levels. When occupancy is below projected levels, the HVAC system is inefficient, moving more air than required and consuming more energy than necessary.
Some HVAC systems have been developed to control the airflow entering buildings. However, these systems do not accurately control volumetric airflow or are very large, expensive, and require large amounts of roof space. The larger systems use long flow straighteners to achieve laminar airflow and accurately measure the airflow entering a building.
Other systems typically use pressure transducers in conjunction with variable opening dampers to control airflow. In these types of systems, the opening of the variable opening damper is adjusted to achieve a particular pressure as indicated by the pressure transducer, which approximates a particular airflow. However, these systems are based on assumptions and approximations that cause the system to be unreliable for accurate control of the airflow. For example, these systems incorrectly assume a laminar flow pattern through the damper and in the vicinity of the pressure transducer, which introduces control inaccuracies that increase as the damper closes and the amount of turbulent airflow increases. Furthermore, these systems approximate the relationship between the damper opening and the volume of air flowing through the damper as a linear relationship, which introduces additional control inaccuracies that also increase as the damper closes.
Consequently, there is a need for improved methods and apparatuses for providing and controlling the ambient air delivered to a building by an HVAC system.
Certain features of embodiments of the present invention address these and other needs and provide other important advantages.
Some or all of these features may be present in the independent or dependent claims which follow herein, but should not be construed to be a limitation unless expressly recited in a particular claim.